I have tried my own freshly-created project, the C++/CLI sample included with NUnit and the one from CodeProject How to use NUnit to test native C++ code using Visual Studio 2008sp1 with NUnit 2.5.5 as well as 2.4.8. I installed 2.4.8 just on C:\ in case there was something weird about paths with spaces such as Program Files (x86).

I have no problems with a C# sample using NUnit.

in NUnit GUI, all of these C++/CLI projects encounter the same problem, on attempting to open the projects.

I'd really like to use NUnit but for now have had to go back to standard Microsoft tests

I had what sounds like the same problem but via the NUnit console. It turned out that the NUnit console was JIT compiling to x64 rather than x86 and my C++/CLI assembly was x86. They need to be the same to work so I used the x86 specific version of the console. See http://www.nunit.org/index.php?p=nunit-console&r=2.4.8.